Waves and Interference Questions Flashcards
Define superposition and coherence
- when 2 or more waves overlap the resultant displacement is the vector addition of the separate waves
- 2 waves that have a constant phase difference and the same frequency and wavelength
State and explain what 2 changes you would expect in a system if each of the slits was made narrower but separation remained the same
- less bright in general due to smaller amplitude of emitted waves
- brighter in the centre as the single slit diffraction pattern dominates
Describe and explain the effect on fringe appearance of : reducing separation of the double slits but keeping slit width constant
- same brightness
- larger separation gives larger fringe widths
Describe and explain the effect on fringe appearance of : making each of the double slits wider but keeping fringe separation constant
- same fringe width -brighter fringes
- less fringes due to less diffraction and as such less overlap
Derive the diffraction grating equation
-think about the triangle and understand yourself
Discuss the 2 types of pulse broadening
modal dispersion: light rays enter the fibre at different angles, and so take different paths. As such some rays will reach the end of the fibre before others and cause pulse broadening. This can be fixed by using a single-mode fibre that only has 1 path.
material dispersion: light consists of different wavelengths that travel at different speeds in the fibre and causes some wavelengths to reach the end of the fibre faster than others. Using monochromatic light can stop this
bonus: signals can be absorbed by the fibre which causes a loss in amplitude of the signal
The figure below shows a diagram of apparatus used to demonstrate the formation of interference fringes using a white light source in a darkened room. Light from the source passes through a single slit and then through two narrow slits S1 and S2.
Describe the interference pattern that is seen on the white screen. (2)
- central white fringe
- fringes each side showing range of colours / spectrum
- red is the furthest and violet the closest to the centre
The figure below shows a diagram of apparatus used to demonstrate the formation of interference fringes using a white light source in a darkened room. Light from the source passes through a single slit and then through two narrow slits S1 and S2.
A filter transmits only green light of wavelength λ and red light of wavelength 1.2λ
This filter is placed between the light source and the single slit.
Describe the interference pattern now seen on the white screen.
Use a calculation to support your answer. (4)
- the central fringe is a mixture of green and red / two wavelengths
- red and green fringes are seen on either side
- spacing of green finges is less than that of red fringes
- spacing of red fringes is 20% greater than green
- 6th green fringe overlaps with 5th red fringe, 1.2 x 5 = 6 x 1
A student decides to use the apparatus shown in the figure to determine the wavelength of red light using a filter that transmits only red light.
The student suggests the following changes:
- decrease slit separation s
- decrease D, the distance between the slits and the screen.
The student decides to make each change independently.
Discuss the effects each independent change has on the interference pattern, and whether this change is likely to reduce uncertainty in the determination of the wavelength. (6)
The following statements may be present for decreasing slit separation s:
Fringe separation increases
Uncertainty in measuring fringe separation will
decrease
and as this is needed to measure wavelength,
uncertainty in wavelength
measurement will decrease
The following statements may be present for smaller D:
Uncertainty in measuring D would increase
Fringe separation would also decrease
so uncertainty in measuring fringe separation would increase
Both are required to find wavelength so uncertainty in finding wavelength would increase
Figure 2 shows the appearance of the fluorescent screen when the electrons are incident on it.
Explain how the pattern produced on the screen supports the idea that the electron beam is behaving as a wave rather than as a stream of particles. (3)
Particle behaviour would only produce a patch/circle of light /small spot of light or Particles would scatter randomly ✔
Wave property shown by diffraction/ interference ✔
Graphite causes (electron)waves/beam to spread out /electrons to travel in particular directions ✔
Bright rings/maximum intensity occurs where waves interfere constructively/ are in phase ✔
An electron beam is incident on a thin graphite target that behaves like the slits in a diffraction grating experiment. After passing through the graphite target the electrons strike a fluorescent screen.
Explain how the emission of light from the fluorescent screen shows that the electrons incident on it are behaving as particles. (3)
Electrons must provide enough (kinetic) energy ‘instantly’ to cause the excitation✔
Idea of light emission due to excitation and de-excitation of electrons/atoms ✔
Idea of collisions by incident electrons moving electrons in atoms between energy levels/shells/orbits ✔
Light/photon emitted when atoms de-excite or electrons move to lower energy levels ✔
Explain why the signal strength falls to a minimum between B and C, and between C and D. (3)
path difference for two waves ✔
Allow ‘waves travel different distances’
Condone out of phase
gives rise to a phase difference ✔
if phase and path confused only give 1 for first 2 marks
Destructive interference occurs ✔
allow explanation of interference
The intensity of the waves passing through each slit is the same.
Explain why the minimum intensity between C and D is not zero. (3)
Intensity decreases with distance ✔
One wave travels further than the other ✔
Amplitudes/intensities of the waves at the minimum points are not equal ✔
The microwaves from the transmitter are polarised. These waves are detected by the aerial in the microwave receiver (probe). The aerial is a vertical metal rod.
The vertical aerial is placed at position B and is rotated slowly through 90° until it lies along the direction AE.
State and explain the effect on the signal strength as it is rotated. (3)
The signal decreases/becomes zero ✔
The waves transmitted are polarised ✔
zero when detector at 90° to the transmitting aerial/direction of polarisation of wave ✔
Explain how the use of the single slit in the arrangement above makes the light from the two slits sufficiently coherent for fringes to be observed. (2)
single slit acts as a point / single source diffracting / spreading light to both slits ✓
OR
the path lengths between the single slit and the double slits are constant / the same / fixed ✓